1. Field of the Invention
The present invention relates to heat-dissipating substrates suitable for semiconductor devices. In particular, the present invention relates to a heat-dissipating substrate comprising a composite material containing a first composition primarily composed of aluminum and a second composition primarily composed of silicon carbide and/or silicon, a method for making the heat-dissipating substrate, and a semiconductor device including the heat-dissipating substrate.
2. Description of the Background Art
Recently, the integration density of semiconductor devices has increased markedly, and heat generated in the semiconductor devices has also increased exponentially. Thus, heat-dissipating substrates used in semiconductor devices must have high heat dissipation and satisfactory matching of thermal expansion coefficient with semiconductor devices and components for the semiconductor devices.
Furthermore, lighter, thinner, and more compact semiconductor devices are developed; hence, heat-dissipating substrates must be thinner and more compact. Also, heat-dissipating substrates have been required to have various complicated shapes, and these heat-dissipating substrates need to be produced with high dimensional precision and low cost.
A known material for such heat-dissipating substrates is a composite material comprising a metal having a low thermal expansion coefficient and a high thermal conductivity, i.e., tungsten (W) or molybdenum (Mo), and another metal having a high thermal conductivity, i.e., copper (Cu). However, the Wxe2x80x94Cu and Moxe2x80x94Cu composite materials are expensive and heavy. When a heat-dissipating substrate made of such a known composite material is mounted onto a plastic motherboard, the motherboard and ball grid are easily deformed or damaged.
Under such circumstances, light ceramic materials having high thermal conductivity such as aluminum nitride (AlN) and silicon carbide (SiC) have been developed as materials for heat-dissipating substrates. However, ceramic materials have a disadvantage of high process cost since they cannot be easily processed. Countermeasure materials are aluminum (Al) and its alloy, which have high thermal conductivity and are light; however, these have low hardness and are easily damaged.
A variety of Al-based alloys and composite materials have been investigated. For example, a composite material of Al and silicon (Si) or a composite material of Al and a ceramic having high thermal conductivity is used for a heat-dissipating substrate. For example, Japanese Examined Patent Application Publication No. 63-16458 discloses a composite material of Al and Si (Alxe2x80x94Si composite material). Japanese Unexamined Patent Application Publication Nos. 1-501489, 2-243729, 9-157773, 10-280082, and 10-335538 disclose composite materials of Al and SiC (Alxe2x80x94SiC composite materials). Composite materials containing Al, Si, and SiC are also known.
However, since these composite materials contain hard SiC and Si particles, much time is required for finishing processes such as cutting and grinding, and high load is applied to the processing tools, which results in quick wear of the tools. Such high load applies considerable stress to the surfaces of the composite materials and causes deformation of the composite materials such as curvature. As described above, thinner substrates have been developed recently. Such thinner substrates are easily deformed by high load during the finishing process. Since both soft Al particles and hard SiC particles are present in the processed surface, the hard particles are readily scratched off by processing tools and the resulting cavities are filled with deformed Al particles. Furthermore, the surfaces processed with worn tools become more uneven and have low dimensional accuracy.
Japanese Unexamined Patent Application Publication No. 10-280082 discloses a method for making an Alxe2x80x94SiC composite material having a predetermined shape without processing such as cutting and grinding. In this method, a green compact having a shape substantially similar to the final shape is prepared, and surfaces of the green compact are covered with specific layers of silicon oxide or the like to prevent liquation of Al after firing. However, the formation of the specific layers on the surfaces of the compact is very troublesome.
In a thin metal substrate composed of Cu or Al, a shallow recess for mounting a semiconductor device is generally formed by chemical etching or corrosion. For example, WO99/0959 discloses an example of chemical etching for making a multichip module. However, chemical etching is predominantly applied to soft metal materials.
Japanese Unexamined Patent Application Publication No. 10-42579 discloses etching of Alxe2x80x94Si and Alxe2x80x94SiC materials for sliders, which are quite different from semiconductor devices. The abrasion resistance of the slider is improved by causing hard particles to protrude 2 xcexcm or less from the surface by slightly etching the Al matrix after finish-machining the surface by cutting or polishing. This technology, however, does not take account of deformation of the material during cutting and polishing.
Various types of semiconductor packages for higher performance and advanced capabilities are developed. For example, packages of flip-chip mounting types shown in FIGS. 7 and 8 are employed for increased I/O terminals. In these drawings, reference numerals 1a and 1b denote heat-dissipating substrates, reference numeral 2 denotes a semiconductor device mounted on the heat-dissipating substrate, reference numeral 3 denotes a multichip-type wiring layer, reference numeral 4 denotes a ball grid (connection terminal). The heat-dissipating substrate 1a shown in FIG. 7 constitutes a lid whose peripheral frame is directly connected with the wiring layer 3. The heat-dissipating substrate 1b shown in FIG. 8 is of a flat plate and the heat-dissipating substrate 1b and the wiring layer 3 are hermetically sealed with a stiffener 5.
In compliance with the requirement of high performance, packages of flip-chip type, the number of the wiring layers 3 is increased. On the other hand, since the total thickness of the package is specified by a JEDEC (Joint Electron Device Engineering Council) standard, the heat-dissipating substrates 1a and 1b must be thinner so that the package complies with the standard. However, the above-described Alxe2x80x94SiC and Alxe2x80x94Si heat-dissipating substrates easily undergo deformation such as curvature as the thickness is reduced by cutting or grinding. As for the lid-type heat-dissipating substrate 1a shown in FIG. 7, in particular, it is difficult to further reduce the thickness thereof while maintaining high dimensional accuracy.
When the heat-dissipating substrate is used in a package, its product name and lot number are printed on a surface of the heat-dissipating substrate. Thus, the surface must be conditioned so that the print is clear. However, identification of printed data has been difficult in the case of the Alxe2x80x94SiC and Alxe2x80x94Si composite materials, because they exhibit dark or low gray metallic luster, although the color varies depending on the composition of the materials.
An object of the present invention is to provide a thin heat-dissipating substrate which comprises an Al-based composite material composed of Alxe2x80x94SiC, Alxe2x80x94Si, or the like and which does not undergo deformation such as curvature and has a surface that allows data printed thereon such as a product code and lot number to be clearly identified.
A heat-dissipating substrate according to the present invention is made of a composite material comprising a first composition primarily composed of aluminum and a second composition primarily composed of silicon carbide and/or silicon, and has a recess in one of the two main faces thereof, in which the maximum amplitude of the fine unevenness in the depth direction of the surfaces excluding the recess portion is smaller than the maximum length in the depth direction of the composite particles comprising the first composition and the second composition or the particles of the second composition that are exposed at the surface of the same main face.
Preferably, the two main faces have a roughness Ra of 0.2 to 2 xcexcm in terms of the roughness Ra defined in JIS B 0651. Preferably, the two main faces excluding the recess portion are gray or light gray without exhibiting metallic luster.
A method for making a heat-dissipating substrate according to the present invention comprises the following steps: a step of preparing a composite material comprising a first composition primarily composed of aluminum and a second composition primarily composed of silicon carbide and/or silicon; a first etching step of forming a recess in one of the main faces of the composite material by chemical etching; and a second etching step of forming fine unevenness on the main faces by chemical etching such that the maximum amplitude of the fine unevenness in the depth direction of a main face is smaller than the maximum length in the depth direction of composite particles composed of the first composition and the second composition or particles composed of the second composition, which particles are exposed at the surface of the main face.
In the method of making a heat-dissipating substrate according to the invention, the chemical etching in the first etching step is performed with an alkaline or acidic aqueous etching solution containing 30 to 50 percent by mass of etchant and the chemical etching in the second etching step is performed with an alkaline or acidic aqueous etching solution containing 3 to 30 percent by mass of etchant. Preferably, the second alkaline or acidic aqueous etching solution further contains another etchant of an inorganic copper salt or chloride wherein the total etchant concentration in the solution is in the range of 3 to 50 percent by mass. The above-mentioned alkaline or acidic aqueous etching solutions may further contain hydrogen fluoride.